This invention relates to protection circuits and, more particularly, to a hammer protection circuit for automatically disconnecting a power supply from a hammer actuating coil in order to inhibit current flow through the coil when such current flow is either greater than a predetermined level or present for greater than a predetermined time period.
U.S. Pat. No. 3,589,282 discloses a hammer protection circuit which automatically disconnects a power supply from a plurality of hammer actuating coils whenever, during a period when the hammers are not being fired, i.e. when current above a predetermined threshold level is normally not supposed to flow through the coils, a current above such threshold level is monitored through any of the coils. This circuit suffers from the disadvantage of being protective only during the period following or preceeding a hammer firing period and not during such time period. Thus, if the current flow through any of the coils is excessive during a hammer firing period, the circuit will not disconnect the power supply until the end of the normal hammer firing period. Furthermore, if the logic that generates the signal representing the end of the hammer firing period fails for some reason and such signal is not generated, the power supply will remain connected even though unwanted current may still be flowing through one or more of the coils. This protection circuit is thus totally dependent upon the proper generation of a signal representing the end of a hammer firing period in order to be operative.
It would be desirable, therefore, to provide a hammer protection circuit which automatically disconnects a power supply from a hammer actuating coil either when the current flow through the coil is excessive, whether during a hammer firing period or not, or when such current flow is present for greater than a predetermined time period, e.g. is present after cessation of a normal hammer firing period.